Electrophotographic dry developers are divided into one-component developers, comprising a toner itself containing a binder resin having dispersed therein a colorant, and two-component developers comprising a toner and a carrier. In carrying out copying using either type of developer, an electrostatic latent image formed on a photoreceptor, etc. is visualized with the developer and transferred to paper. Toner remaining on the photoreceptor is then wiped off by cleaning.
Accordingly, a dry developer is required to satisfy various conditions in the copying step, particularly in the development step or cleaning step. That is, a toner should be used not in the form of agglomerates but in the form of independent particles. To this effect, it is required that the toner should have sufficient fluidity and that the flow characteristics or electrical characteristics of the toner should not be subject to variation with time or change in environmental conditions such as temperature and humidity.
In addition, the toner in a two-component developer is required to cause no filming phenomenon, i.e., adhesion of a toner, onto the surface of carrier particles.
Further, the residual toner on a photoreceptor should be easily wiped clean by means of a cleaning part, such as a blade or a web, without to scratching the photoreceptor.
For the purpose of meeting these requirements, it has been proposed to add to a one-component or two-component developer various external additives for improving fluidity, durability or cleanability, for example, inorganic powders (e.g., silica), organic powders (e.g., fatty acids, fatty acid metal salts, and derivatives thereof), and fluorine-containing resin powders.
Addition of the inorganic powders, such as silica, titania, and alumina, considerably improves fluidity but is liable to cause scratches on the surface of the photoreceptor because of their hardness. It easily follows that toner particles adhere to the scratched part of the photoreceptor.
On the other hand, regenerated paper has been steadily extending its use with the aim of resources-saving. In general, regenerated paper generates much paper dust, and the paper dust tends to enter the gap between a photoreceptor and a cleaning blade, causing cleaning defects, such as black streaks.
In order to overcome these problems, JP-A-60-198556 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes external addition of a fatty acid metal salt, and JP-A-61-231562 and JP-A-61-231563 propose external addition of a wax.
Any of these external additives proposed as a lubricant has a large particle size of from 3 to 20 .mu.m. Accordingly, they should be added in a considerable amount to be made efficient use of. Besides, although these lubricants are effective in the initial stage, they themselves undergo filming, failing to form a uniform lubricating film, causing image defects, such as white spots and faint image.
JP-A-2-89064 suggests adding hydrophobic hard fine particles to a toner so that a photoreceptor is abraded by the hard fine particles to prevent toner filming. While effective to prevent filming, the hard particles cause wear on the surface of the photoreceptor, resulting in a serious reduction in durability of the photoreceptor. A cleaning blade is also worn out by the hard fine particles.